The long-term goal of this research project is to elucidate the molecular mechanisms that regulate the formation of synaptic contacts. Synapses are a central component in the formation and the functioning of the nervous system and become affected in a variety of mental and neurological disorders. Therefore the discovery of the molecules and mechanisms that participate in the establishment of synaptic connections will contribute to our understanding of the formation of neuronal circuits, the causes of mental and neurological disorders, and may identify potential targets for therapeutic interventions. Synapse formation is a well-programmed developmental process that involves a variety of cell-cell interactions carried out by distinct groups of molecules, which are required for the recognition of a postsynaptic target, the stabilization of the contact between synaptic membranes, and the functional coupling between synaptic compartments. The present proposal focuses on the identification and characterization of proteins capable of inducing the formation of synaptic contacts between neurons during embryogenesis. Our central hypothesis is that the expression of membrane tethered or secreted ligands by the presumptive postsynaptic neuron are required to induce the differentiation of the presynaptic terminal and the establishment of a synaptic contact. For this reason, we carried out a genome-wide search for gene-transcripts that become expressed in postsynaptic neurons during the different phases of synapse development. This analysis led to the identification of a group of proteins containing immunoglobulin (Ig) domains, leucine-rich repeats (LRR), or other protein interacting domains that are substantially up-regulated during the initiation phase of synapse formation, and which have the molecular features of cell surface ligands suggesting that these proteins may possess synaptogenic activity. In this two-year project we propose 1) to test experimentally the synaptogenic activity of this subset of proteins up-regulated during the initiation of synapse formation, 2) to examine whether their combinatorial expression is needed to trigger synapse formation, and 3) to test whether cell surface expression of GPI- linked cell adhesion molecules regulates the surface expression levels of nicotinic acetylcholine neurotransmitter receptors. To this aim, in vitro cell assays will be used to determine the synaptogenic activity of these proteins by evaluating presynaptic terminal differentiation and cell surface protein expression levels. The identification of proteins which are sufficient to induce synapse formation in vitro will be further studied in the future to determine whether these proteins are necessary for the initiation of synapse formation in vivo and to examine the molecular mechanisms involved. PUBLIC HEALTH RELEVANCE: The long-term goal of this project is to elucidate the cellular and molecular mechanisms that participate in the assembly of synaptic contacts. The present proposal focuses on the identification and characterization of genes and proteins that participate in the initiation phase of synapse formation. The central hypothesis is that proteins expressed on the surface of presumptive postsynaptic neuron are necessary for inducing the differentiation of a presynaptic terminal at the site of contact. The importance of understanding the molecular mechanisms of synapse formation is underscored by the fact that synapses are the center piece for neuronal communication and become affected in a variety of mental and neurological disorders including, intellectual developmental disabilities, autism, and schizophrenia. Thus, the studies proposed in this application will lead to a better understanding of the molecular mechanisms that participate in the formation of synaptic connections, to the elucidation of the causes of mental and neurological disorders, and to the identification of therapeutic strategies.